I'm still collecting all the parts for the next major step for my bronco, full widths. I got a set of '79 F150 axles. Along with the axles I have a C6 and 203/205 double I am going to stuff in between the frame rails. Since the drive line will lengthen then I will stretch my wheel base. My engine is a efi 302 with a RV cam.

My question is what would be the best gear ratio for the axles? For the time being I will be running 35 inch tires but soon to step up to 37s when money permits. My bronco sees mostly trails and rock crawling with a little around town driving.

I have a similar setup. Given the doublers I went 4.56 and would do the same on yours. Without overdrive i was spinning the engine faster than I would have liked on the highway. You'll have plenty of tcase gears to still crawl without 4.88s.

The problem will be the 302. they just don't have the nads to overcome crappy gearing. you are covered in low range. But in reality I would go 4.88's with the jump to 37's. I have run 37's, 4.88's, c6, 205. I worked and the 203 would have made it even better. With the 4.88's if all fails you could hope on the highway and run 65(3k rpm'ish). the 302 will want that rpm. Most wheeling the 2:1 low of the 205 and 4.88's would wheel great.

I'd certainly target making your setup happy on the highway since your doubler will cover you offroad. Have you ran your numbers thru any of the gear calculators?

My current set up is 302/ZF-5/Atlas 3.8 to 1 and I run 4.88's with 37's. I can run 75 mph on the highway with this set up. My crawl ratio is probably too low in first but I just use 2nd and have plenty. Since there is no overdrive in the C6 probably want to confirm with one of the calculators.

I like this page for gearing calculations.

https://www.novak-adapt.com/knowledge/gearing/

Thanks Everyone for the input!